Hydrostatic transmissions are well known and generally may include a hydraulic pump and a hydraulic motor. The hydraulic pump and the hydraulic motor may be arranged as separate components or may be combined together in an integral unit. Axial swashplate type hydraulic piston pumps are frequently used in many such hydrostatic transmissions. Such pumps generate a pump action by causing pistons to reciprocate within a piston bore, with reciprocation of the pistons being caused by a swashplate that the pistons act against as a cylinder barrel containing the pistons rotates. Pump fluid output flow or displacement for each revolution of the barrel depends on the bore size and the piston stroke as well as the number of pistons that are utilized. The swashplate can pivot about a swashplate pivot center or axis, and the swashplate pivot angle determines the length of the piston stroke. By changing the swashplate angle, the pump displacement can be changed as is known in the art.
With the swashplate at its extreme pivot angle relative to the axis of rotation of the barrel, a maximum fluid displacement is achieved. When the swashplate is centered at a right angle relative to the axis of rotation of the barrel, the pistons will not reciprocate and the displacement of the pump will be substantially zero. In some axial swashplate type piston pump designs, the swashplate has the capability of crossing over center which results in the pump displacement being generated at opposite ports. In an over center swashplate axial piston pump, each system port can be either an inlet or an outlet port depending on the pivot angle of the swashplate. Over center axial swashplate piston pumps are widely used in hydrostatic transmissions, to provide driving in both forward and reverse directions.
One use for hydrostatic transmissions is zero turn vehicles such as zero turn lawn mowers. A separate over center swashplate axial piston pump may drive a hydraulic motor and wheel on each side of the vehicle. When the swash plate angles of the two pumps are equal and the output flow rotates the wheels in the same direction at the same speed, the vehicle travels in a substantially straight line path in either the forward or the reverse direction. When the swash plate angles of the two pumps are not equal and the output flow rotates the wheels in the same direction but at different speeds, the output flow rotates one wheel faster than the other so that the vehicle will turn. When one of the pumps is rotating its associated wheel in one direction and the other pump is rotating its associated wheel in the other direction, the vehicle will make a zero radius turn. An operator interface allows the vehicle operator to control the swashplate angles of the separate over center swashplate axial piston pumps, to control straight line or turning or zero radius turns for the vehicle.
In such pumps and systems and methods and vehicles, a technical problem is to precisely control the swash plate angle of the pump in response to operator input. This is necessary, for example, to drive the vehicle in a straight line path in the forward or reverse direction, or to make smooth turns including zero turns, or to drive in confined areas under precise control. A further technical problem is to achieve this precise control with good operator feel characteristics, in a repeatable manner, with multiple inputs, and at a relatively low cost.